Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-09-07
2004-05-11
Niebling, John F. (Department: 2812)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000
Reexamination Certificate
active
06734924
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having circuits structured by electric field effect transistors (FETs), for example, thin film transistors (TFTs), and to a method of manufacturing the semiconductor device. The present invention relates, for example, to a semiconductor device, typically a liquid crystal display panel, and to an electronic device in which such a semiconductor device is mounted as its component.
Note that, throughout this specification, the term electro-optical device indicates general devices for performing shading display by changing an electrical signal, and that liquid crystal display devices and display devices using electroluminescence (EL) are included in the category of electro-optical device.
Note also that, throughout this specification, the term element substrate indicates general substrates on which active elements such as TFTs and MIMs are formed.
2. Description of the Related Art
Techniques of structuring thin film transistors using semiconductor thin films (having thicknesses on the order of several nm to several hundred nm) formed on a substrate having an insulating surface have been focused upon in recent years. The thin film transistors are being widely applied to electronic devices like ICs and semiconductor devices, and in particular, their development has accelerated rapidly as switching elements of liquid crystal display devices.
Liquid crystal display devices are known to be roughly divided into active matrix types and passive matrix types.
A high-grade image can be obtained with active matrix liquid crystal display devices using TFTs as switching elements. Active matrix applications are generally to notebook type personal computers, but they are also expected to be used in televisions for a home and in portable information terminals.
The active matrix liquid crystal display devices are generally driven by line inversion drive. With the line inversion drive, for example source line inversion drive, the polarity of voltages applied to adjacent source lines differs, as shown in
FIGS. 37A and 37B
, and the polarity of the voltage applied to each source line changes each frame.
FIGS. 37A and 37B
show the polarity of voltages applied to pixels during the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent source line is referred to as the source line inversion drive. Drive in which the polarity of the voltage differs for each adjacent gate line is referred to as gate line inversion drive.
FIG. 10
shows schematically a cross section of a pixel portion of a liquid crystal display device. An electric field formed between pixel electrodes
102
a
and
102
b
formed on a substrate
101
, and an opposing electrode
103
formed on an opposing substrate
104
, as shown in
FIG. 10
, is referred to as a vertical direction electric field
105
in this specification. Further, an electric field formed between the adjacent pixel electrodes
102
a
and
102
b
is referred to as a horizontal direction electric field
106
in this specification.
Liquid crystals in the vicinity of the pixel electrodes orient themselves along the horizontal direction electric field if the line inversion drive is performed, the liquid crystal orientation in edge portions of the pixel electrodes becomes nonuniform, and disclinations develop. In order to obtain a good quality black level, light shielding films for covering the disclinations are necessary. However, the aperture ratio drops if the disclinations are covered by the light shielding films. It is necessary to come up with a scheme in which a good quality black level can be obtained, and as little disclination as possible develops when displaying a high aperture ratio, bright image. Note that, in this specification, liquid crystal orientation irregularities developing due to differences in the direction of the pre-tilt angle, and differences in the twist direction, at liquid crystal orientation film interfaces are referred to as “disclinations”. Further, regions having different brightnesses produced due to an irregular orientation state of the liquid crystals when a polarization plate is formed is referred to as “light leakage”.
In particular, the occupied ratio of pixels in which disclinations and light leakage developing due to horizontal direction electric fields is large enough that it cannot be ignored in liquid crystal display devices in which the pixels are formed at a very fine pitch, such as that of a projecting liquid crystal display device. Further, these disclinations and light leakage are expanded when projected onto a screen with the projecting liquid crystal display device, and therefore whether or not the light leakage and disclinations can be suppressed is vital in maintaining contrast.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an element structure such that liquid crystal disclination and light leakage can be stopped in an active matrix liquid crystal display device.
The following measures are taken in order to solve the above stated problems with the conventional technique.
Overlapping edge portions of pixel electrodes with predetermined height convex portions.
FIG. 2
is a cross sectional diagram of a simulation model. The present invention utilizes moving disclinations and light leakage, which are caused when a voltage is applied to liquid crystal
202
, to edge portions of pixel electrodes by forming edge portions of pixel electrodes
203
a
and
203
b
on a first substrate (not shown) so as to overlap with convex portions
204
formed on a level surface as shown in FIG.
2
. An opposing electrode
201
is formed in an opposing substrate
207
.
Note that, in this specification, the convex portions are formed selectively below the pixel electrodes. Regions in which the pixel electrodes overlap with upper edge portions of the convex portions are referred to as first regions (a) of the pixel electrodes. Regions in which the pixel electrodes are formed in side portions of the convex portions are referred to as second regions (b) of the pixel electrodes. Regions in which the pixel electrodes are formed on a level surface, and which contact the second regions of the pixel electrodes, are referred to as third regions (c) of the pixel electrodes.
Further, the height (h) of the convex portions is the maximum value of the length of a vertical line formed from the upper edge portions of the convex portions to the level surface on which the convex portions are formed.
In addition, a cell gap (d) is the distance from the opposing electrode formed on the opposing substrate (second substrate) to the third region of the pixel electrode.
An inter-pixel electrode distance (s) is the distance between the first regions of mutually adjacent pixel electrodes.
Conventionally, if there are convex portions in the orientation surface of liquid crystals, then the orientation of the liquid crystal is disordered and light leakage develop at the convex portions, and therefore it is thought that a liquid crystal orientation surface should be as level as possible. However, the applicants of the present invention found that when the first regions of the pixel electrodes formed on the convex portions having a predetermined height, and the second regions of the pixel electrodes formed on the side portions of the convex portions having a predetermined height, are present, simulation results on the liquid crystal orientation show that liquid crystal orientation irregularities caused by the horizontal direction electric field in driving a liquid crystal display device can be reduced. Specifically, locations at which disclinations and light-leakage appear are in the edges of the pixel electrodes during black display.
This phenomenon is explained by the schematic diagrams of
FIGS. 1A
to
1
C which show the principles of the present invention. The liquid crystal orientation method is taken as a TN method.
FIGS. 1A
to
1
C show liquid crystal orientations when driving a liquid cry
Hamamoto Yuriko
Hirakata Yoshiharu
Satake Rumo
Cook Alex McFarron Manzo Cummings & Mehler, Ltd.
Niebling John F.
Semiconductor Energy Laboratory Co,. Ltd.
Stevenson André C.
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